I see nothing wrong with lots of players leaving guilds of many years because of GBG. For mobile players, it didn't much matter if a guild was doing GvG or not. Many mobile players were happy with an active GE guild that traded fair.
With GBG, many mobile players now find themselves in the wrong guild. Maybe the guild is serious about GBG and they're not. Maybe they're serious about GBG and the guild isn't. I expect to see lots of guilds churn players as players choose new guilds more aligned with their goals. That's my plan.
The way they're built and with the number of advancements I can contribute each day, both of my cities should be in a guild that's playing in Platinum league. In one world, the gold guild I'm in is kicking ass and taking names and should easily be in Platinum in a few more rounds. In the other gold guild, it's like pulling teeth to get members to participate, even the leaders. I'm hoping they'll step up, but it doesn't appear they will.
I'd love to stay, help members improve, help the guild grow, and fight our way up to Platinum, but I don't see that level of commitment from enough members and seemingly no interest in that from leadership. If that's the case, the best way to change my guild, will be to change my guild.
As far as those leaving the game entirely, we hear those threats every time there's a major change. How many actually leave, I can't tell you. What I can tell you is that it doesn't phase Inno, because both the number of FoE players, and more importantly Inno revenues continue to grow. Also curious how many long term guild members might tell the old guild they're leaving the game, instead of telling them they're leaving for another guild. You never know, you know?